Various components of aerospace systems must be insulated because they may be exposed to elevated service temperatures or large temperature gradients. Ceramic insulation is applied to aircraft engine and engine exhaust components such as nose cones, firewalls and exhaust ducts to protect them and surrounding structure. Ceramic insulation is also applied to the outer surfaces of space vehicles (i.e., "space shuttle tiles") to protect the vehicle against the elevated temperatures and significant temperature gradients experienced during takeoff and re-entry.
Bendig, U.S. Pat. No. 5,041,321, describes a method of making rigid, low-density, fibrous ceramic insulation by forming a slurry of ceramic fibers; molding the slurry to form a soft felt mat; drying the mat; incrementally introducing a sol into the mat; and gelling the sol upon each infusion. The incremental addition of the sol is accomplished through a multiple impregnation or infusion technique in which a small amount of sol is initially infiltrated into the mat, is gelled, and is cured to stabilize the mat dimensionally. Stabilizing allows handling and further processing of the mat. The partially completed mat is strengthened to its final form by infusing, gelling, and curing addition sol in the mat. Usually several infusion cycles are required to achieve adequate strength. Each infusion increases the density of the mat. This technique cures the mat to a rigid shape without appreciable shrinking of the resultant structure. Density of the final product can be controlled. Typical densities are 8-25 lb/ft.sup.3.
Despite its many advantages, the method disclosed in U.S. Pat. No. 5,041,321 has difficulty repeatedly producing relatively thick insulation products that have uniform structure and density. To be practical, thick insulation products should be relatively free of cracks, but cracks are a common problem with the method of U.S. Pat. No. 5,041,321. Therefore, processing improvements are desirable to improve the quality of these products and to reduce their cost of manufacture.